I have a 30g tank that I plan on using as a sump. I haven't put in the dividers for media and return pump yet, so I could get a larger tank for a sump if I needed to. I plan on running two Quiet One 4000 pumps which pump 1017gph each. At about 5 feet of head, they will combine for about 1400gph total (this is the exact same setup on my current 125g tank). That's 11x turnover rate per hour on the tank, which should be more than enough. The 30g sump ought to hold enough media to filter the tank for 2 rays, I think, especially with that flow rate. Like I said, I can probably get a 55g instead if it's considered absolutely necessary. I'll have 2x 300w heaters in the sump.
Whatever rays I get shouldn't be in the tank more than 12 months at most, as I plan on buying or building a 300g tank within that time frame. As long as I get smaller rays (younger), they should do fine in it with just a pair in that time, right?